When Margaret I became ruler of Denmark, Norway, and Sweden (1387–88), it was understood that she should, at the first convenient opportunity, provide the three kingdoms with a king who was to be her nearest kinsman; and in 1389 she proclaimed her sister’s grandson, Erik of Pomerania, king of Norway. In 1396 homage was also rendered to him in Denmark and Sweden, Margaret reserving to herself the office of regent during his minority. To weld the three kingdoms still more closely together, Margaret summoned a congress of the three councils of state (the Rigsraads) and other magnates to Kalmar in June 1397; and on Trinity Sunday, June 17, the joint coronation of Erik united the kingdoms.

The proposed act of union divided the three Rigsraads, but according to modern scholarly opinion the document embodying the terms of the union never got beyond the stage of an unratified draft. Margaret objected to the clauses that insisted that each country should retain exclusive possession of its own laws and customs and be administered by its own dignitaries, for she believed that such policies would tend to prevent the complete amalgamation of Scandinavia. She avoided every appearance of an open rupture, however, and succeeding monarchs also avoided stirring up the issue.

The Kalmar Union lasted until Sweden rebelled and became independent in 1523, under King Gustav I Vasa. At the same time, Norway sank to the status of a Danish province (1536).